A new computer system for STEM3, designed and constructed by T. Kycia, employs a 486 PC and several custom cards for scan generation, microscope control, data acquisition, parameter monitoring and display of images. The custom cards are very simple and inexpensive to copy using standard components. The user software and status display panel duplicate all the functions of the VAX 11-750 system operating STEM1 but with additional features. A second PC, connected to the first via Ethernet, houses commercial data acquisition cards for all functions not essential for microscope operation: monitoring beam current, temperatures, vibration, and stray field, as well as the electrical precision of high voltage, lens, and deflection supplies. Also, the second PC has commercial cards for driving motors and monitoring interlocks. All relevant information and any fault conditions are displayed on the monitor panel. A copy of this system is being assembled for STEM1 to replace the obsolete VAX 11-750. All of our instruments, image analysis stations, AutoCAD, and office computers are connected to each other and to the BNL Network via Ethernet. A Sun workstation, designated as our local server, houses an 8GByte image archive disk, a 1 GB scratch disk, and a 5 GB magnetic tape drive, all used for images, database information, and software. Image transfer over the network takes roughly 1 sec. Backup is now on 8mm tape with each tape holding ~ 6 months data from STEM1, but we are converting to standard 650 MByte CD for long-term archiving and image distribution. Image distribution is also available on Internet and data from both STEMs is available as it is being acquired. Three PC's are set up for Photoshop/SigmaPlot/Canvas figure preparation with output on the Biology Dept. TekPhasar color printer.